High efficiency photographic emulsions are generally recognized to be silver bromoiodide emulsions. The presence of iodide ions in the silver bromide crystal structure of the grains is recognized to produce latice irregularities that enhance latent image formation (observed as increased imaging sensitivity) on exposure to electromagnetic radiation. Silver halide photography is benefited from the development of tabular grain silver bromoiodide emulsions. As employed herein, the term "tabular grain emulsion" designates any emulsion in which at least 50% of the total grain projected area is accounted for by tabular grains. Whereas tabular grains have long been recognized to exist to some degree in conventional emulsions only recently has the photographically advantageous role of the tabular grain shape been appreciated. The silver bromoiodide tabular grain emulsions exhibiting particularly advantageous photographic properties include the high aspect ratio tabular silver halide emulsions having a ratio of greater than 8:1 and the thin intermediate aspect ratio tabular silver halide emulsions having an average aspect ratio in the range down to 5:1. Typical of preparation of tabular grain emulsions are those techniques described in U.S. Pat. Nos. 4,433,048--Solberg et al and 4,439,520-- Kofron et al.
It is known that the halide composition of emulsion grains as precipitated can be modified by halide conversion, i.e., the displacing of more soluble silver halide salt by less soluble salt, such as described by Allentoff et al, U.S. Pat. No. 3,477,852. The halide conversion can be limited to specific position on the silver halide grains, such as described in Maskasky U.S. Pat. No. 4,142,900, European Patent Application EP 273 429, and in European Patent Application EP 273 430. Attention is directed to Section 1 of Research Disclosure 308119, December 1989, pages 993-995.
While the bromoiodide emulsions, particularly those with tabular grains, have been successful in producing silver halide grains suitable for photographic use, there remains a need for improved grains. Further, it is desirable that some emulsions previously not utilizable, be improved by treatments after precipitation such that they become more useful.